Ethylene belongs to the group of plant hormones of natural origin. In a concentration of 0.02-1 ppm ethylene is capable of inhibiting the growth of the stem, exerting defoliant effect and accelerating the ripening of fruits [OSBORNE, J. Plant Growth Regulators, S.C.I. Monograph No. 31 p. 236, (1968)].
The use of gaseous ethylene is accompanied by a number of technical difficulties and consequently has been limited in practice. The preparation of the complex of ethylene and .alpha.-cyclodextrin was described by CRAMER and HENGLEIN in 1957 [Chem. Ber. 90, 2572 (1957)].
A complex of 0.7 mole of ethylene per 1 mole of .alpha.-cyclodextrin is known from the prior art (KOKAI No. 75 58226); according to this patent application the use of the complex results in the shortening of the ripening period of the tomato by about 4 days. Since the complex contains about 0.7 mole of ethylene per 1 mole of cyclodextrin and the percentage ethylene content of the complex is only 2% the complex is only of theoretical value and be used in practice in this form.
In practice when ethylene activity is required compounds are used from which, on decomposition, ethylene is evolved on the surface of the plant or in the interior of the cells. 2-chloroethylphosphonic acid is such a compound of the most widespread application.
On decomposition of 2-chloroethylphosphonic acid with water, hydrochloric acid, phosphoric acid and gaseous ethylene are evolved. Decomposition takes place only at a pH value above 3.5. The commercial products are mixtures which contain various byproducts and water. The most frequently used composition is an aqueous solution comprising approximately 40% of 2-chloroethylphosphonic acid.
When sprayed onto the plants in a suitable dilution 2-chloroethylphosphonic acid enhances the intensity of respiration, increases the activity of chlorophyllase and thus decreases the chlorophyll content of the leaves and exerts defoliating activity.
The pH value of the commercially available 2-chloroethylphosphonic acid concentrates is about 2 in order to inhibit the decomposition of the compound. This low pH value is not advantageous from the point of view of application (corrosion) nor is it advantageous for the reason that the 2-chloroethylphosphonic acid penetrating the plant does not exert a local effect only, but on absorption due to the systemic effect the activity can be observed in the following year and it damages the buds.